NOTTINGHAM, England – Using experimental techniques, researchers at Northumbria University, Newcastle, England, and Nottingham Trent University, Nottingham, England, have made the first-ever direct observation of the elusive dewetting process, which takes place when a liquid film retracts to form a bead-shaped drop. The achievement could now spark a new line of research and lead to breakthroughs involving the use of liquids, such as better coatings and more effective self-cleaning surfaces.

Dewetting is the opposite of spreading, a familiar process that can be observed day-to-day, such as when a drop of oil is placed on the surface of a pan. The liquid initially has a bead-like shape, and it slowly spreads to form a thin film. The opposite process, called dewetting, occurs when a liquid film retracts from a solid to form a bead-shaped drop, which can be observed when a wet window is left to dry up. Despite its apparent simplicity, the direct observation of the full dewetting of a droplet into a single drop had remained elusive and difficult to achieve until Northumbria and Nottingham Trent’s experiment.

In a recent paper in the journal Science Advances, the research team came up with an ingenious solution to this problem. Using a novel method known as dielectrowetting, they exploited the electric properties of liquids to force a liquid to coat a solid surface using an applied voltage.
Professor Glen McHale, Pro-Vice Chancellor (Engineering and Environment) at Northumbria University and Professor of Applied and Material Physics, said, “Our experimental setup opens up the possibility of preparing liquid shapes in a very controlled manner, which then dewet. This can lead to new methods for liquid manipulation in technologies such as coating and self-cleaning surfaces.”

By embedding very thin patterned electrodes in the solid and carefully arranging them into a circular pattern, the team achieved the formation of a thin circular liquid film. By switching off the voltage, they revealed, for the first time, the full dewetting process of the liquid film back to a bead-like drop shape.
Professor Carl Brown, Coordinator of the Nottingham Trent University Engineering Research Unit and Professor of Physics in the School of Science and Technology, said, “At first sight, one might have expected that dewetting is just the time reversal of spreading. Surprisingly, we found that dewetting not spreading in reverse. Instead of a smooth sequence of drop-like shapes, the dewetting film forms a rim at its own edge, which retracts at constant speed for most of the dewetting process.”

To understand this behavior, the team used a combination of theory and numerical simulations to rationalize the experiments. Dr Rodrigo Ledesma-Aguilar, from Northumbria, said, “Both the simulations and the theory support that the liquid tends to adopt the closest local equilibrium shape it can during dewetting. This explains the smooth rim shape, which survives for most of the process.”

Michael Newton, Reader in Experimental Physics in the School of Science and Technology at Nottingham Trent University, added, “Our method can be used to learn more about the underlying physics behind other dewetting phenomena such as condensation, evaporation and droplet rebound. These processes are critical for applications such as fog-collection, coating and lubrication. The technique developed can also be used for characterizing liquid properties when only small volumes are available.”